The present invention relates to electrolytic capacitors and, more particularly, to low cost porous anode capacitors having improved first charge properties and cathode area enhancement.
Liquid or gel electrolyte porous anode type capacitors require cathodes of very high capacitance density (microfarads per square inch). This is necessary because the effective anode area is very large and because the total capacitance of the device is dependent upon both anode capacitance or area and cathode capacitance or area. The very large effective area of the anode structure is realized by sintering very fine particles of valve metals to form a porous body. To obtain best utilization of the effective anode area, it is desirable to provide a cathode of greatly expanded surface area. As cathode area or capacitance becomes increasingly larger and approaches a theoretically infinite value, the total unit capacitance approaches the capacitance of the anode.
Increased effective cathode surface area (cathode enhancement) may be achieved by depositing a conductive layer of particles on the inner surface of the capacitor cathode casing. In currently commercially available porous anode capacitors, the cathode-enhancement material usually employed is platinum, palladium or rhodium. Typically, these metals are deposited on silver or silver alloy casings as known in the art with reasonably good results. It has been found, however, that platinum, palladium, and rhodium yield unsatisfactory results when deposited upon other cathode metals such as nickel and titanium. In particular, it has been found that the deposited layers lacked the tenacity required to withstand substantial vibration during operation and that capacitors employing these cathodes exhibited relatively inferior electrical properties.
It was taught in U.S. Pat. No. 3,531,693, issued to Buice on Sept. 29, 1970, and assigned to the present assignee, that particles of ruthenium can effectively be used to enhance the cathode surface areas of capacitors with nickel casings. The capacitor taught by Buice was inexpensive due to its replacement of the expensive silver cathode casing with a casing of nickel. Furthermore, the capacitor works well in many applications. However, the capacitor taught by Buice has not found wide acceptance as a timing capacitor due to its first charge characteristic. That is, after the capacitor has remained idle for a period of time, a somewhat longer than normal period of time is required to recharge the capacitor. This effect is particularly noticeable at temperatures exceeding 85.degree. C. Consequently, timers employing the capacitor can suffer from some inaccuracy if they are used only sporadically.
It is, therefore, an object of this invention to provide a capacitor that is relatively inexpensive but which exhibits good first charge characteristics, thus permitting its use as an inexpensive timing capacitor.